Zinc dihydrocarbyldithiophosphates are well-known lubricating oil additives. When added to oil in small amounts they impart both antioxidant and antiwear properties to the oil. The major use for such oils is in crankcase oils for internal combustion engines.
Zinc dihydrocarbyldithiophosphates are made by the reaction of an alcohol or phenol with phosphorus pentasulfide to form the corresponding O, O-dihydrocarbyldithiophosphoric acid which is then reacted with zinc oxide to form zinc dihydrocarbyldithiophosphate. The reaction with zinc oxide proceeds quite well with dialkyldithiophosphoric acids, but with diaryldithiophosphoric acids the reaction with zinc oxide is much slower and it is very difficult to obtain a "sweet" product -- i.e., a product having a pH over about 5.0. In Blaha, British Pat. No. 1,289,199 (U.S. Pat. No. 3,562,306), there is provided an improvement in the process whereby zinc nitrate, chloride or sulfate are added as a reaction promoter.